Donald Farr
Former Editor, Mustang Monthly
March 26, 2010
Photos By: The Mustang Monthly Archives

Mike Eaton and I are walking through the show field at the Mustang & Ford Roundup at Silver Springs, Florida, when he spots a '66 Mustang that is obviously sitting like a grasshopper with rearend jacked up at the rear. "That car has definitely got the wrong springs on it, unless the guy wanted it that way," Mike says while pointing out the gap between the rear tire and the wheel opening. "It sits about 2- to 2 1/2 inches too high in the rear so it doesn't look right. That's what can happen when you buy the 'one-size-fits-all' springs. The springs fit, but the car doesn't sit right. A lot of people don't know that in 1966 there were four different rear leaf springs for Mustangs."

And Mike should know. His grandfather started Eaton Detroit Spring after buying the aftermarket division of Detroit Steel Products, a manufacturer of original equipment springs, in 1937. Mike began working there in 1977 and bought the company in 1985. As a spring manufacturer in a facility located on Michigan Ave. in Detroit, near the old Tiger Stadium, Eaton Detroit Spring has over 24,000 original spring blueprints for cars back to 1893, all catalogued by OE numbers, vehicle, and application.

The '65-'73 Mustangs used a combination of front coil springs and rear leaf springs. Eaton Detroit Spring is one of the few companies that manufacture both, either by following factory blueprints or custom. You can find application guides for '65-'73 Mustang coil and leaf springs at the Eaton website, www.eatonsprings.com. Follow the "Tech info" and "Downloads & Literature" links.

Our discussion with Mike revolved around the replacement of factory production springs, not aftermarket performance applications, although Mike says Eaton Detroit Spring can supply factory-spec Heavy-Duty or Competition springs for cars that came with the standard suspension.

MM: We see a lot of Mustangs sitting too low in front or too high in the rear, like that '66 out in the show field. Springs have a lot to do with that, right?
Eaton: A car's stance makes or breaks the vehicle. And the only thing that changes a car's ride height is the springs. It's not like a headlight bulb or a windshield wiper; springs are specific to the vehicle, determined by the equipment on that vehicle and the total weight. That's why all the information we need-body style, engine, etc.-is so critical. In addition to setting the ride height, springs also support the car's weight and provide ride comfort.

MM: So choosing the right springs for a Mustang is a lot more complicated than just finding springs that will fit?
Eaton: Picking the right spring is very important. You need the year, make, engine, body style, whether it's air-conditioned or not, because all those things affect the weight of the vehicle. The rear leaf springs for all these years are the same length but they have different arches and different rates. Rate is how strong the spring is. For example, 85 pounds means it takes 85 pounds to deflect the spring one inch. Mustangs were arranged from around 85 to 153 pounds for the '69 Boss, which was the only time that spring was used. Ford used 32 different rear springs for the '65-'73 Mustangs. In '65, there were four different leaf springs; by '69 there were 12.

MM: Does equipment make the biggest difference in spring selection?
Eaton: Yes. With the front coil springs, you had different springs for air-conditioned cars because of the weight of the A/C. If you calculate how much weight is hanging over the front end, you can see that A/C adds about 300 pounds. The actual parts may only weigh 60-65 pounds, but they're out in front of the springs so you can figure about 300 pounds. Not so true for aftermarket air because it can be lighter, but with factory air it's about 300 pounds.

The transmission can make a difference in the front springs due to the weight, automatic versus manual. Even the seats-you'd think the weight difference between a bench seat and two buckets seats wouldn't make a lot of difference, but apparently it did. The location of the battery made a difference in '69 rear springs. Is the weight of the battery under the hood or in the trunk?

Also, the two-barrel or four-barrel engine in '71 made a difference. The 302 and 351 two-barrels used the same rear springs but when it went to the four-barrel, prior to Oct. 9, it used the same rear spring. After Oct. 9, the rear springs changed. That's common when it comes to springs.

There were only two or three different front coil springs for '65-'66 Mustangs, but it went to five for '67, seven for '68, and eight different front coil springs on '69-'70 Mustangs. Then in later years, they started standardizing things and didn't go so crazy. In '73, that generation of car was phasing out so they started settling down with the number of different springs.

MM: What about build date?
Eaton: When the car was built can make a difference if Ford made some change that calls for different springs. The rates and number of leaves are the same, but for some reason Ford made a change in the specification of the spring. For '69 Mustangs with the 390 engine, there was a change on Sept. 10, 1968. Prior to Sept. 10, they had an Improved Handling package, but after Sept. 10, they called it Competition Handling. The spring rate on the Improved Handling-technically heavy-duty-was 114 pounds, but after Sept. 10 the rate went up to 135 pounds.

MM: What if someone wants to step up to Competition or heavy-duty suspension springs? If they order for their car and their equipment, the car should sit OK, right?
Eaton: If they call us with a '73 convertible and the specifications call for an 85-pound spring but they want the Boss 302 spring that's 153 pounds and they want it sitting at the stock ride height, that's a piece of cake. We can do that for them. Chances are, if they order the regular Boss 302 spring, the car would not sit right. But if we know that they want that spring for their particular car, we'll make that spring so the car will sit right.

MM: So you can make custom springs?
Eaton: As a manufacturer, we can make anything. We have countless number of thousands of custom blueprints. Probably 85 percent of our production is custom. It's non-stop applications, so if someone wants their car to ride higher or lower, softer or harder, we can do that.

Only a handful of companies make coil springs. Most are high-volume guys and they won't even talk to you about custom springs. We're a low-volume manufacturer so we can make custom springs, providing the tooling is in place and we're making something like it already. In that case, making a one-off, custom spring is no big deal.

Leaf spring tooling is a piece of cake. For coil springs, we have about 450 sets of tooling; we stock or create somewhere around 650 part numbers. And out of those 650, we can alter each one of them a minimum of six different ways. The last time we counted leaf springs, we have something like 1,800 different patterns. Out of those, you can generally make somewhere between six and 12 different types of springs of each pattern, so we can make literally thousands of different springs. If you ask one of the other companies about a custom spring, they'll probably give you our phone number.

MM: How can Mustang owners order your springs?
Eaton: They can call us directly, or a number of Mustang vendors, like National Parts Depot, YearOne, and others, handle a broad selection of our springs instead of the one-size-fits-all. Some vendors offer four springs at the most, where Ford might have had 32. The one-size-fits-all springs will fit on the car because they're all the same dimensions and the same number of leaves, but they can vary the ride height plus or minus three inches.

MM: What should owners tell you when ordering springs?
Eaton: We need the year, make, model, body style, engine size, A/C or not, and any and all modifications made to the vehicle. Remember, springs support weight. And all those parts that are or aren't there equal weight. If you take a big-block engine and put on aluminum heads, aluminum intake, and headers, you're now talking about the weight of a small-block. So if you don't tell us about those changes and we send you big-block springs, you're not going to be a happy camper. Once we know all the information, we can design the spring. The other thing we need to know is how you want it to ride and how you want it to sit as compared to a stock vehicle.

MM: What if someone orders springs, providing all the correct information, and the car still sits too high or low?
Eaton: They should call us. We'll have one question right off the bat: "Is the car finished?" Sometimes they say, "No, I don't have the engine and transmission in it." Geez, they're missing about half of the weight. Sometimes they respond, "But me and my six buddies stood on it." That still doesn't matter because the weight isn't distributed the way it should be. So we tell them to finish the car, drive it like they stole it-accelerate it, brake, hit potholes-to let the springs work and settle to where they're supposed to be. If you do that and it's still not settling at the right ride height, we'll take care of it. We can tell you where to measure, and with those measurements we'll know what adjustments to make. Second phone calls are rare.

MM: You mentioned that springs can settle. Is that something that owners should expect with new springs?
Eaton: Once a spring is installed, will the vehicle settle? Yes. Coil springs will settle about a quarter of an inch and leaf springs might settle a half an inch. That's once they've been used. Everything we sell has been shot-peened to relieve stress and tension. We take the steel when it comes out of heat-treating and bombard it with hundreds of thousands of little steel balls at high velocity for about a minute to a minute and 10 seconds. We like to say the molecules are unhappy when they come out of heat-treating. They're under tension. Shot-peening turns the molecules into what we call happy molecules-they're relaxed and ready to go to work. That not only relieves some of the tension in the springs, it also increases spring life up to 10 times.

Just like anything else, you get what you pay for. A lot of the one-size-fits-all springs are made from 1095 steel. Our springs are made out of SAE 5160 steel-that's high-quality and high-cost steel, made just for springs. It costs more than springs made out of 1095 steel, which has no memory or life. But some companies make springs out of it; they just don't last.

MM: We hear about owners cutting the front coils to lower the front of a Mustang for better handling. Is that something owners should avoid?
Eaton: Cutting coils is fine. You have to look at the ends of the coil-how are the ends of the coils shaped? You can tell a square end if you stand the coil up and it stays standing up. That's a square-end coil. You can't cut that because it has to sit in the spring pocket, making contact all the way around. The other kind is a tangential coil; it just twists off into space. If you try to stand that coil up, it will fall over. That spring can be cut. Mustangs have a square end on the bottom and tangential on the top. Cut it with a hacksaw; don't use a torch. A torch heats the spring red-hot-hot enough to cut steel, as they say-and some place down the length of that coil, it's going to cool off. And where it goes from red-hot to cool, it can become brittle and break in that area.

MM: Obviously, Eaton Detroit Spring can supply a proper spring with the lowered ride height.
Eaton: Certainly. If they want to cut them, cut 'em. It works. Just don't cut too much off.

MM: What about the spacers they sell at the auto parts stores for raising the front of a car with old springs?
Eaton: The ones you screw in, then hit a good chuck-hole and watch them go bouncing out? They do have aluminum solid spacers-you take the coil out and it fits underneath. Those are fine.

MM: On rear leaf springs, is there a choice of eye location?
Eaton: We do that too. It changes the ride height of the car. Typically, Mustangs had the standard eye, with the eye at the top. When you reverse the eyes, bring them halfway down, you change the ride height half the diameter of the eye. If you reverse them both, you change the ride height the average diameter of the eye. If you reverse one eye, you change the ride height half the diameter of that eye. You can really begin to refine the ride height.

MM: What about lowering blocks for the rear leaf springs?
Eaton: The leading cause of handling problems is the use of blocks, whether it's for raising or lowering. You're increasing that angle, the fulcrum point. Go back to your basic physics. You increase the leverage, so the harder you pull that spring away from the spring seat from the centerline of the axle, the more leverage you create and the more axle windup you get. And the more squirrelly the car is.

MM: Any common mistakes that people make when ordering springs?
Eaton: Not being truthful about what they have. Over the last three years, 99.7 percent of errors were because people weren't honest about their cars. And the other 0.3 percent we screwed up. But the most common mistake is that people don't tell us what they really have. There's nothing to be embarrassed about. If you've got a six-cylinder, don't tell us it's got a 429. And don't believe what your buddy told you about what his friend did. We're the experts. We'll point you in the right direction. We want the second phone call from a customer to be for another order, not a complaint.

Spring Terms
Rate:
Amount of weight required to deflect the spring one inch. The lower the rate, the softer the spring.

Load: Amount of weight the spring is designed to carry at a certain height. Also called design load or load rate.

Free arch: Amount of arch in a leaf spring with no load on it.

Loaded height: Measured the same as free arch except the spring is under load.

Stepping: Distance from the end of one leaf to the end of the adjoining leaf. Stepping controls the shape and strength of a spring when it's under load.